Introduction

Our planet is experiencing a massive decline in biodiversity, which is largely due to human activities, and which could ultimately lead to the sixth extinction of animal and plant species on the Earth.1, 2 This global change in the environments can affect ecosystem functioning and lead to significant disruptions of ecosystems, which may threaten the human sources of livelihood and the current way of life. Loss of biodiversity is a global concern and may lead to a variety of possible adverse consequences for the human population.3 The reasons underlying such loss of biodiversity are complex and have been suggested to be largely linked to the consequences of growing urbanisation and industrialization, climate change, increasing pollution, and increasing utilization of chemicals, which have impact on the environment and microorganisms with which humans coevolve.4, 5
Biodiversity was defined by von Hertzen et al. 1 as “The variability among living organisms from all sources, including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part this includes diversity within species, between species and of ecosystems ”. Haahtela6 also suggested that micro-organisms play a key role in the link between biodiversity-related environmental changes and human health. Loss of biodiversity and disappearance of natural habitats may reduce the diversity of environmental microbiota, i.e., the biodiversity of the outer layer.6 According to Haahtela6, humans are protected by two nested layers of biodiversity, i.e. microbiota of the outer layer and of the inner layer. The outer layer is dependent on the environment we live in (including soil, natural waters, plants, and animals); and the inner layer inhabits the human body (including gut, skin, and airways) and is dependent on colonization from the outer layer. Furthermore, the diversity and composition of human microbiota (i.e., inner layer) can also be influenced by environmental exposures (i.e., outer layer).7-9
In 2011, von Hertzen et al. 1 proposed that loss of biodiversity also leads to immune system dysfunction and increases the risk of chronic inflammatory diseases, including asthma and allergies, chronic obstructive pulmonary disease, type 1 diabetes, obesity and inflammatory bowel diseases, and could therefore have important public health implications. The biodiversity hypothesis proposed by von Hertzen et al. 1 is consistent with the observed declining trends of biodiversity indices, such as Waterbird Population Status Index (WPSI) and Living Planet Index (LPI), and with increasing trends in the prevalence of asthma and allergic rhinitis since the 1970’s.5 Rapidly declining biodiversity may be a contributing factor to another global megatrend, the rapidly increasing prevalence of allergies and other chronic inflammatory diseases among urban people.5 Increasing evidence suggests that the diversity of human microbiota influences the risk of asthma and allergies.10 Changes in the development of microbiota, evidenced by low gut and airways microbiota diversity in infancy, has been associated with the development of atopy and asthma later in life.10, 11 The biodiversity hypothesis has stimulated substantial research on the role of biodiversity for the risk of developing asthma and allergic diseases, but the results have so far been inconsistent.10, 11 This heterogeneity in results may be related to different definitions and measures of biodiversity, timing and duration of exposure, as well as differences in duration of the follow-ups. Therefore, this systematic review and meta-analysis aims to summarize the current knowledge on the role of biodiversity in the development of asthma, wheezing, and allergic sensitization.